Practice: Oxidative Phosphorylation 3 - Online Tutor, Practice Problems & Exam Prep

The electron transport chain generates reactive oxygen species, specifically superoxide radicals (O₂^-), which are converted to hydrogen peroxide by superoxide dismutase. This hydrogen peroxide is further reduced to water by glutathione peroxidase, utilizing NADPH. The overall standard redox potential of electron transport is calculated as 1.14 volts. The energy released during electron transfer from cytochrome a₁ to a₃ is determined using the equation ${\mathrm{\Delta G}}^{=}-nFE$, resulting in -5,790 joules per mole.

Practice

The reactive oxygen species made in quinones during electron transport are released into the mitochondrial matrix as superoxide radicals. So these are O2- which are called superoxide radicals and you might recall that these are actually converted into hydrogen peroxide by superoxide dismutase. We talked about this first when we looked at the pentose phosphate pathway because this hydrogen peroxide is then converted to water by glutathione peroxidase, and that enzyme actually uses glutathione, oxidizes it, and then glutathione has to be reduced by NADPH, which is one of the uses of NADPH and part of the reason it's generated in the pathway.

Now, given the standard reduction potentials here, what is the overall standard redox potential of electron transport? So, hopefully, you realize that NADH is going to actually be the beginning of electron transport, right? So we have to reverse this equation. So we are going from NADH→NAD++H++ 2 electrons, meaning that our reduction potential is actually going to be positive 0.32. And the reason this is significant is our overall equation is the potential for oxidation plus the potential for reduction. So we had 2 reduction potentials given to us but we needed this to be flipped so that it would be the oxidation potential. So that makes our overall equation 0.32+0.82=1.14 volts, right? Because we are dealing with volts here.

Now, what is the energy released as one mole of electrons moves between cytochrome a3 and a1 in complex IV? So here, we actually are looking for the energy and we're given the reduction potential. So we are going to need to use this equation, ΔG = -nFE. Here, ΔG is obviously the change in Gibbs free energy, n is the number of moles, and F is Faraday's constant which is equal to 96,500 joules per volt times mole, or sometimes it's put as 96.5 kilojoules per volt times mole, right? Be careful about that. Determine our reduction potential, right? Well, where are the electrons coming from, and where do they go? Cotton-eyed Joe. Tell me that. So they come from a1 and they go to a3, so a1 is going to be oxidized, meaning this has to be flipped. We get that ΔG = -1. Right? N is 1, just 1 mole. Times 96,500 joules per volt times mole, and E here is going to be equal to 0.35-0.29 (both in volts). So our answer, our overall answer is -5,790 joules per mole or -5.79 kilojoules per mole if you prefer. Same difference.

Alright, let's flip the page and finish up with some photophosphorylation questions.

Here’s what students ask on this topic:

What are reactive oxygen species and how are they managed in the electron transport chain?

Reactive oxygen species (ROS) are highly reactive molecules containing oxygen, such as superoxide radicals (O₂^-). In the electron transport chain, these ROS are generated in quinones and released into the mitochondrial matrix. Superoxide radicals are converted into hydrogen peroxide (H₂O₂) by the enzyme superoxide dismutase. Hydrogen peroxide is then reduced to water (H₂O) by glutathione peroxidase, which uses glutathione (GSH) and oxidizes it to glutathione disulfide (GSSG). The oxidized glutathione is subsequently reduced back to GSH by NADPH, which is produced in pathways like the pentose phosphate pathway.

How is the overall standard redox potential of electron transport calculated?

The overall standard redox potential of electron transport is calculated by summing the reduction potentials of the involved reactions. For example, if NADH is oxidized to NAD⁺ + H⁺ + 2e^- with a reduction potential of -0.32 V, and another reaction has a reduction potential of 0.82 V, the overall redox potential is calculated as follows:

$0.32+0.82=1.14 V$

This results in an overall standard redox potential of 1.14 volts.

What is the energy released as one mole of electrons moves between cytochrome a1 and a3 in complex IV?

The energy released during electron transfer between cytochrome a1 and a3 in complex IV can be calculated using the equation:

$\Delta G=-nFE$

where ΔG is the change in Gibbs free energy, n is the number of moles of electrons (1 mole), F is Faraday's constant (96,500 J/V·mol), and E is the difference in reduction potentials (0.35 V - 0.29 V = 0.06 V). Substituting these values:

$\Delta G=-1 \times \mathrm{96,500} \times 0.06=-\mathrm{5,790} J/\mathrm{mol}$

Thus, the energy released is -5,790 joules per mole, or -5.79 kJ/mol.

What role does NADPH play in the management of reactive oxygen species in the mitochondria?

NADPH plays a crucial role in managing reactive oxygen species (ROS) in the mitochondria by maintaining the reduced state of glutathione (GSH). When superoxide radicals (O₂^-) are converted to hydrogen peroxide (H₂O₂) by superoxide dismutase, glutathione peroxidase further reduces H₂O₂ to water (H₂O). This process oxidizes glutathione (GSH) to glutathione disulfide (GSSG). NADPH, generated in pathways like the pentose phosphate pathway, is then used by glutathione reductase to reduce GSSG back to GSH, ensuring a continuous supply of reduced glutathione to detoxify ROS.

How does the pentose phosphate pathway contribute to oxidative phosphorylation?

The pentose phosphate pathway contributes to oxidative phosphorylation by generating NADPH, which is essential for maintaining the reduced state of glutathione (GSH). Glutathione is crucial for detoxifying reactive oxygen species (ROS) produced during oxidative phosphorylation. Specifically, superoxide radicals (O₂^-) are converted to hydrogen peroxide (H₂O₂) by superoxide dismutase, and then to water (H₂O) by glutathione peroxidase, which oxidizes GSH to glutathione disulfide (GSSG). NADPH is used by glutathione reductase to reduce GSSG back to GSH, thus supporting the cell's antioxidant defenses.